Conventional outdoor lighting controllers are generally composed of a light sensing element such as fabricated from cadmium sulfide for sensing variation of daylight level and a bimetallic thermal switch in the power supplying circuit for the light being controlled. A series circuit comprising the light sensing element and a resistive wire is connected in parallel to a power source supplying the light being controlled, wherein the resistive wire is used to heat the bimetallic thermal switch when the light sensing element senses increase of daylight level and exhibits a low resistance.
As the light sensing element senses daylight waning at dusk, the light sensing element exhibits a high resistance approximately on the order of 100 mego-ohms. Because there is only a small amount of electric current flowing through the series circuit comprising the light sensing element and the resistive wire, the bimetallic thermal switch in the power circuit is thus released to turn on the light being controlled.
As the light sensing element senses daylight increase at dawn, the light sensing element exhibits a low resistance as stated earlier whereby the electric current flowing through the series circuit comprising the light sensing element and the resistive wire increases to a great extent. The bimetallic thermal switch in the power supplying circuit is thus heated by the resistive wire to switch off the light being controlled.
While the stated conventional controller is quite simple in construction, it suffers from a plurality of defects which were left unsettled heretofore. The defects inherent in the stated conventional controller are herein briefly described as follows:
(1) The series circuit comprising the light sensing element and the resistive wire is directly connected to a 110 VAC or 220 VAC power source or even a higher one requiring that a light sensing element with higher power rating must be used. The resultant cost is thus much higher as compared with the light sensing element used in the controller disclosed by the present invention;
(2) To permit operation over a wide ambient range of temperature, the stated conventional controller is additionally provided with a compensating device formed of a metallic plate which unavoidably suffers from fatigue in the case of continuous operation;
(3) The stated conventional controller is generally installed with the light sensing device covered by a transparent cap. It is quite obvious that the accumulation of dust on the outer surface of the transparent cap will prevent some daylight from penetrating the cap. As a result, the weakened light level inside the cap will make the controller initiated earlier than it should be at dusk and disabled later than it should be at dawn. As known, street lamps in heavier air polluted areas are generally controlled by man; this is quite uneconomical and the controlling is not always duly performed. An outdoor lighting controller is therefore provided according to the present invention to remove defects inherent in the conventional outdoor lighting controllers.